This invention relates to the technology of evaluating the granularity or image quality of digital radiation image. In particular, the invention relates to a method and an apparatus for evaluating the granularity or image quality of digital radiation image with x-rays by means of a digital radiographic apparatus.
Various diagnostic techniques using x-rays and other radiations are currently practiced in the medical field. In those techniques, x-rays or other radiations that have passed through the object are detected to generate an analog image for use in medical diagnosis, as exemplified by time-honored chest radiography.
In recent years, various digital radiation images such as those produced by DR (digital radiography), CT (computed tomography) and MRI (magnetic resonance imaging) are presented on CRT displays or output from printers for use in clinical settings to diagnose diseases. Among these digital radiation images, those produced by DR with CR (computed radiography) systems using x-rays are most popular in the medical field.
In a CR system, an x-ray detector coated with a stimulable phosphor (i.e., a stimulable phosphor sheet) is contained in a light-shielding case (so-called cassette) and exposed to x-rays that have passed through the object. The exposed stimulable phosphor sheet is taken out of the cassette and the entire portion of its recording surface is scanned with laser beam optics. Upon excitation by the laser beam, the stimulable phosphor in the sheet gives off an emission of stimulated light in proportion to the absorbed dose of x-rays. The emitted light is collected by a focus guide, detected with a photosensor and later objected to digital conversion to produce digital image data. The digital image data is then objected to various processes such as gradation treatment and spatial frequency enhancement and the processed image is presented on a CRT display or output from a printer.
Since the digital image produced from the CR system is principally composed of digital data, it can be modified in a desired manner by means of various filters such as the above-mentioned gradation treatment and spatial frequency enhancement. Take, for example, medical x-ray image; to insure safety for the patient, the dose of exposure to x-rays should be held to a minimum and the resulting digital image is objected to gradation treatment so that images of comparable gradation and density can be produced independently of the exposed dose of x-rays.
The granularity of digital image is ascribable to three major causes, fluctuation in x-rays, fluctuation in the light emitted from a stimulable phosphor, and the structure of a stimulable phosphor sheet. Digital image can be recorded with a wide range of exposure to x-rays and a specific dose is chosen for the particular object of recording. Since the granularity and image quality of digital image are largely dependent on the exposed dose of x-rays, they must be tested and evaluated under constant exposure conditions. However, applying the correct and constant dose of x-rays on every occasion of testing or evaluation is difficult and the difference in exposed dose leads to an error in the data of testing or evaluation.
Under these circumstances, the CR system requires that the degree of granularity of radiation image and its image quality be constantly monitored and controlled so that the appropriate and refined diagnosis can be performed consistently on the basis of the recorded image. To meet these requirements, the granularity and image quality of the radiation image obtained by the CR system have to be known for predetermined doses of exposure to x-rays. Conventionally, exact matching to predetermined doses of exposure to x-rays has been impossible and the resulting errors have caused errors in evaluation of the granularity and image quality of the radiation image produced by the CR system. This difficulty in achieving correct control over the values of evaluation of the granularity and image quality of radiation image from the CR system is not limited to x-rays and is also encountered in the image of other radiations such as xcex1- and xcex2-rays.
The present invention has been accomplished under these circumstances to solve such problems that cannot obtain precisely values of evaluation of the granularity and the image quality of the digital radiation image for predetermined dose exposure to radiation and has as an object providing a method by which the correct values of evaluation of the granularity and image quality of digital radiation image can be obtained for precisely predetermined doses exposed to radiation.
Another object of the invention is to provide an apparatus for implementing the method.
The first object of the present invention can be attained by a method for evaluating a digital radiation image which obtains the digital radiation image from a radiation image recorded with a given dose exposed to a radiation and then evaluates for at least one of granularity and image quality of the thus obtained digital radiation image, comprising the steps of recording previously at least two radiation images with different exposed doses of the radiation, obtaining at least two digital radiation images from the at least two radiation images, determining a value of evaluation of at least one of the granularity and the image quality of the digital radiation image for each of the thus obtained at least two digital radiation images to obtain at least two values of evaluation, constructing from the at least two values of evaluation a correction curve for the value of evaluation to the exposed dose of the radiation and calculating by means of the correction curve the value of evaluation of the at least one of the granularity and the image quality of the digital radiation image obtained from the radiation image recorded with the given dose exposed to the radiation.
In a preferred embodiment, the digital radiation image is a digital radiation image produced by objecting to predetermined processes the radiation image recorded with a phosphor material that emits stimulated light in proportion to the exposed dose of the radiation, the value of evaluation is a square value of RMS granularity or a value of a Wiener spectrum at each frequency for evaluating the granularity of the digital radiation image, the correction curve for the value of evaluation to the exposed dose of the radiation X is expressed as A/X+B (where A and B are constants) and constructed by determining the constants A and B from the at least two values of evaluation.
In another preferred embodiment, the digital radiation image is a digital radiation image produced by objecting to predetermined processes the radiation image recorded with a phosphor material that emits stimulated light in proportion to the exposed dose of the radiation, the value of evaluation is an NEQ value for evaluating the image quality of digital radiation image, the correction curve for the value of evaluation to the exposed does of the radiation X is expressed as 1/(C/X+D) (where C and D are constants) and constructed by determining the constants C and D from the at least two values of evaluation.
In yet another preferred embodiment, the digital radiation image is a digital radiation image produced by objecting to predetermined processes the radiation image formed on a radiation detector coated with a phosphor material that emits stimulated light in proportion to the exposed dose of the radiation, the radiation detector is divided into at least two regions, split recording is performed by applying different exposed doses of the radiation to the at least two regions so as to produce the at least two radiation images with different exposed doses of the radiation.
Preferably, the digital radiation image is used as its image data which have been objected to logarithmic conversion.
The second object of the present invention can be attained by an apparatus for evaluating a digital radiation image produced by objecting to predetermined processes a radiation image which has been recorded on a radiation detector coated with a phosphor material that emits stimulated light in proportion to an exposed dose of a radiation, comprising a digital image acquiring device that records at least two radiation images with different exposed doses of the radiation either on one and same radiation detector by split recording or on separate radiation detectors and which applies predetermined processes on the recorded at least two radiation images to produce at least two digital radiation images, and at least one of: a granularity evaluating device which determines, from each of the at least two digital radiation images acquired by the digital image acquiring device, a square value of RMS granularity or a value of a Wiener spectrum at each frequency for evaluating the granularity of the digital radiation image, constructs from determined values a correction curve A/X+B (where A and B are constants) as a function of the exposed dose of the radiation X, and calculates from the thus constructed correction curve a value of evaluated granularity for a given dose exposed to the radiation, and an image quality evaluating device which determines, from each of the at least two digital radiation images acquired by the digital image acquiring device, an NEQ value for evaluating image quality of the digital radiation image, constructs from determined values a correction curve 1/(C/X+D) (where C and D are constants) as a function of the exposed dose of the radiation X, and calculates from the thus constructed correction curve a value of evaluated image quality for a given dose exposed to the radiation.